Gas streams often carry particulate material. In many instances, it is desirable to remove some or all of the particulate material from a gas flow stream. For example, air intake streams to engines for motorized vehicles or power generation equipment, gas streams directed to gas turbines, and air streams to various combustion furnaces, often include entrained particulate material. The particulate material, should it reach the internal workings of the various mechanisms involved, can cause substantial damage. Removal of the particulate material from the gas flow upstream of the engine, turbine, furnace or other equipment involved is often needed.
The invention relates to polymeric fine fiber compositions used on a enhanced efficiency, (e.g.) HEPA or ULPA substrate. The fine fiber compositions have improved properties that can be used in a variety of applications including the formation of fibers, microfibers, nanofibers, fiber webs, fibrous mats, permeable structures such as membranes, coatings or films. The polymeric materials of the invention are compositions that have physical properties that permit the polymeric material, in a variety of physical shapes or forms, to have resistance to the degradative effects of humidity, heat, air flow, chemicals and mechanical stress or impact.
In making fine fiber filter media, a variety of materials have been used including fiberglass, metal, ceramics and a range of polymeric compositions. A variety of fiber forming methods or techniques have been used for the manufacture of small diameter micro- and nanofibers. One method involves passing the material through a fine capillary or opening either as a melted material or in a solution that is subsequently evaporated. Fibers can also be formed by using “spinnerets” typical for the manufacture of synthetic fiber such as nylon. Electrostatic spinning is also known. Such techniques involve the use of a hypodermic needle, nozzle, capillary or movable emitter. These structures provide liquid solutions of the polymer that are then attracted to a collection zone by a high voltage electrostatic field. As the materials are pulled from the emitter and accelerate through the electrostatic zone, the fiber becomes very thin and can be formed in a fiber structure by solvent evaporation.
Industrial dust collection equipment is also required to remove greater proportions of smaller and smaller particles from process air streams due to increasingly stringent regulatory requirements. Gas turbine intake filtration systems also must remove quantities of very small particles as the presence of such particles can cause irreparable damage to turbine blades. The cleanliness of an environment, the health of its occupants, the effectiveness of industrial processes, the maintenance of industrial equipment, and the overall aesthetics of living require that submicron particulate materials be readily removed by filter from an air stream.
In order to achieve submicron particulate removal from air streams passing through such systems, inertial separators tend to simply place a physical barrier in the path of particulate material that is then knocked from the air stream into a collection bin. Paper bag dust collectors are simply filters based on paper filter technologies in a bag form. Such paper bags are typically simply fit across the air stream for the purpose of separating particulate from the air stream.
Newer filters have been designed with a collection filter or a flat panel or cylindrical cartridges. In these applications, a HEPA filtering material is used. Typically such HEPA Structures include an expanded PTFE (stretched Teflon) layer with a layer of a melt blown fiber combined in a filter construction, or a cellulose filter paper layer with a layer of melt blown fiber combined in a filter construction. These structures are often cleaned simply by rapping the filter or by blowing filter cake or particulate from the filter using compressed air streams.
The filtration efficiency and cleanability of newer filters is important. These filters must be able to remove dust and dirt, but must be easily cleanable without damage to the filter. Often cleaning dirty filters by rapping the filter on a solid object to dislodge dust and dirt can cause the filter media to fail or can cause multilayered elements to delaminate, thus causing the filter to fail through the formation of a pathway for the dust and dirt through the filter structure. Another failure mode occurs when fine dust particles are trapped into the depth of the filter media, such that the dust cannot be dislodged by typical filter cleaning mechanisms, resulting in reduced vacuum power and shorter filter life.
The technology disclosed herein relates to an on-going development of Donaldson Company Inc., of Minneapolis, Minn., the assignee of the present invention. The disclosure concerns continuing technology development related, in part, to the subjects characterized in U.S. Pat. Nos. B2 4,720,292; Des. 416,308; 5,613,992; 4,020,783; and 5,112,372. Donaldson, Inc., of Minneapolis, Minn.; also owns each of the patents identified in the previous sentence and, the complete disclosure of each is incorporated herein by reference.
The invention also relates to polymer materials used as a robust HEPA or ULPA substrate, can be manufactured with improved environmental stability to heat, humidity, reactive materials and mechanical stress. Such materials can be used in the formation of fine fibers such as microfibers and nanofiber materials with improved stability and strength. As the size of fiber is reduced the survivability of the materials is increasingly more of a problem. Such fine fibers are useful in a variety of applications. In one application, filter structures can be prepared using this fine fiber technology. The invention relates to polymers, polymeric composition, fiber, filters, filter constructions, and methods of filtering. Applications of the invention particularly concern filtering of particles from fluid streams, for example from air streams and liquid (e.g. non-aqueous and aqueous) streams. The techniques described concern structures having one or more layers of fine fibers in the filter media. The compositions and fiber sizes are selected for a combination of properties and survivability.
One example of a dust filter vacuum technology using a fine fiber layer in a vacuum bag is Emig et al., U.S. Pat. No. 6,395,046. One example of a filter cartridge in a wet/dry vacuum using expanded PTFE or stretched Teflon is Scanlon et al., U.S. Pat. No. 5,783,086. One example of a cellulose filter paper/melt blown layered filter is the Ridgid-brand VF5000 filter available at Home Depot stores. One example of a scrimmed HEPA filter is the Shop-Vac 903-34-00 HEPA Cartridge Filter available from www.shop-vac.com. Filter materials, such as scrimmed HEPA media often have high efficiency, but often have short lifetimes and can be degraded through water exposure. Expanded PTFE media typically have very high pressure drops and moderate HEPA efficiency. Cellulose filter paper/melt blown layered filters typically exhibit poor filter cleanability.
The market for general-purpose vacuum cleaners and for wet/dry vacuum systems has imposed increasingly high standards of performance for the vacuum cleaners and their filters over recent years. The devices are required to remove greater and greater proportions of smaller and smaller particles from streams obtained by the vacuum cleaner from often-harsh wet or dry environments in the home, garage, basement, shop, yard, and a variety of industrial environments. The increased requirements satisfy needs for improved health, reduced allergies, improved cleanability, reduced ambient particle counts, and other requirements for home, shop, and industrial environments.
A substantial need exists, in light of attempts using prior filter structures, to obtain filtration at moderate to low pressure drop, extended lifetime, filtration ability to remove large amounts of submicron particulate at relatively high flow rates and extended lifetimes. The preferred filter is cleanable mechanically with by rapping on a solid object or by a simple water wash, and can survive repeated use and cleaning cycles.